<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  

 <script>
  let arr = [88,20,10,100,50]
  let ten = 10
   for (let i = 0; i < arr.length; i++) {
    if (arr[i]===10) {
      document.write(i);
    } else {
       document.write('-1');
    }
    
   }
 </script> 
</body>
</html>